In a substrate processing apparatus, a substrate undergoing processing for semiconductor device production, such as a wafer, is carried into a processing chamber via a transfer means such as a transfer arm and a specific type of processing, e.g., etching or film formation, is executed on the wafer having been carried into the processing chamber. As increasingly fine circuit patterns have come to be formed through highly advanced micro-processing technologies in recent years, it has become necessary to position the wafer undergoing processing or inspection along the correct orientation with a high level of positioning accuracy in correspondence to the nano-order device design specifications (e.g., circuit line width of 65 nm). A notch mark such as an indented notch or a linear notch, which is often referred to as an orientation flat, is formed at part of the edge of the wafer, and the wafer is positioned by using this notch mark.
The wafer is usually positioned by, for instance, disposing a light emitting unit and a light receiving unit of a light-transmitting sensor so as to operate across the wafer edge. Light that is radiated toward the wafer edge is transmitted through the edge, and the notch mark is thus detected as the quantity of transmitted light changes at the notch mark.
In recent years, transparent wafers constituted of optical material or transparent material with superior light transmission characteristics and electrical insulation characteristics, such as sapphire, glass and quartz, have become fairly common as alternatives to the conventional silicon wafers. However, unlike a nontransparent wafer constituted of a nontransparent material such as silicon, a transparent wafer allows light to be transmitted over substantially the entire area thereof, and it has been assumed that a notch mark at the wafer edge cannot be detected with the light-transmitting sensor described above.
Accordingly, a light-reflecting sensor, instead of a light-transmitting sensor is used in conjunction with such a transparent wafer to detect the notch mark by radiating light from one side of the transparent wafer and receiving the light reflected from the wafer (see Japanese Laid Open Patent Publication No. H06-085038 and Japanese Laid Open Patent Publication No. H10-163301).
A light-transmitting sensor may be utilized to detect the edge area of a transparent wafer if the wafer has a portion formed along its edge over the entire circumference where vertical light is not allowed to advance straight, e.g., a beveled portion formed as a slanting surface over the entire circumference.
However, since light is transmitted through almost the entire area of a transparent wafer, there is a high likelihood of the light sensor generating saturated data (abnormal data) due to noise light such as disturbance light, regardless of whether the light sensor is a light-transmitting sensor or a light-reflecting sensor. For instance, if a pattern has been formed on the transparent wafer, noise light resulting from light diffraction manifesting at the pattern may be received by the light sensor and saturated data may be generated as a result. The term “saturated data” in this context refers to data indicating the maximum value that the light sensor is capable of detecting.
As described above, saturated data tend to be generated readily when the detection target is a transparent wafer. For this reason, if wafer edge shape data detected by the light sensor include a plurality of sets of saturated data indicating a noise area. A noise area may be erroneously judged to be the notch mark depending upon how the saturated data are distributed. Such an erroneous judgment leads to a problem in that the wafer positioning processing cannot be executed with accuracy.
It is to be noted that this problem may occur when the detection target wafer is a nontransparent wafer, as well as when the target wafer is a transparent wafer. For instance, abnormal data may be generated due to disturbance light or electrical noise when the detection target wafer is a nontransparent wafer, and in such a case, a noise area may be erroneously judged to be the notch mark formed at the wafer, depending upon the extent of variance in the abnormal data.